1. Field of the Invention
This invention relates to a liquid measuring system, and more particularly to a system for repeatedly measuring a selectively fixed amount of liquid. The system for measuring liquid in accordance with this invention is particularly directed to an apparatus for measurement of liquid in which a predetermined amount of liquid is measured by use of a liquid level measuring device comprising a combination of pressure transmitters provided at different levels.
2. Description of the Prior Art
Conventionally, a variety of liquid measuring devices or apparatus as of the float-type, electrical-probe-type, weight-transformer-type and pressure-gauge-type have been used for industrial measurement of liquid. In these types of measuring devices, the level or amount of liquid is measured by use of a liquid level or liquid amount sensing element which is provided in a measuring cylinder and the supply of liquid into the cylinder is stopped by the signal from the sensing element.
The float-type liquid measuring device employs a float with a magnet and a magnetically operable switch provided in the measuring cylinder. When the level of the liquid supplied into the cylinder increases and the float floating on the liquid comes up to a predetermined level, the switch is closed by the magnetic force of the magnet in the float. The supply of liquid into the cylinder is then stopped in response to the closure of the switch. Thus, a predetermined amount of liquid is measured. In this type of liquid measuring device, however, there is a disadvantage in that the position or level of the magnetically operable switch must be changed when the amount of liquid to be measured is to be changed. That is to say, the measuring device of this type requires a movable part and is therefore difficult to design and maintain.
The electrical-probe-type liquid measuring device is particularly used for measuring liquid having electric conductivity. This type of measuring device usually consists of one or two electrodes extending into a cylinder for the purpose of starting and stopping pumps and operating solenoid valves or other types of equipment to control the supply of liquid into the cylinder. When the liquid level comes up to the position of the electrodes which are located at a predetermined level, a solenoid valve which is electrically connected with the electrodes is energized to stop the supply of liquid into the cylinder. Thus, a predetermined amount of liquid is measured. This type of liquid measuring device, however, also suffers from the same defects as those inherent in the foregoing float-type liquid measuring device because of the necessity to change the position of the electrodes when changing the amount of liquid to be measured.
The weight-transformer-type liquid measuring device employs a weight transformer which transforms weight to an electrical value. The total weight of the cylinder and liquid contained therein is measured by the weight transformer. When the total weight reaches a predetermined value, the supply of liquid into the cylinder is stopped. Thus, a predetermined amount of liquid is measured. This type of liquid measuring device suffers from the defect that the range of measurement is reduced by the weight of the cylinder which is measured together with the liquid.
The pressure-gauge-type liquid measuring device employs a strain-gauge-type pressure gauge disposed on the bottom of the cylinder to measure the liquid pressure at the bottom of the cylinder. On the basis of the relationship between the amount of liquid in the cylinder and the liquid pressure at the bottom of the cylinder, the predetermined amount of liquid to be measured can be represented by a predetermined value of liquid pressure measured at the bottom of the cylinder. Therefore, by stopping the supply of liquid into the cylinder when the liquid pressure measured at the bottom of the cylinder reaches said predetermined value, a predetermined amount of liquid is measured. This type of liquid measuring device suffers from the following defect. It is known that the absolute error produced in a strain-gauge-type pressure gauge of a fixed full scale is always constant. Therefore, the relative error increases as the amount of liquid to be measured decreases. Accordingly, the range of measurement is reduced. In other words, in this type of liquid measuring device, it is impossible in the practical sense to measure amounts of liquid which are small with respect to the full scale of the pressure gauge used therein. In order to use this type of liquid measuring device to measure such small amounts of liquid, a pressure gauge of small full scale must be prepared separately.